1. Field of the Invention
The present invention relates to a telecommunication optical cable to be installed in gas pipelines or the like and in particular an optical cable provided with a built-in leakage-detecting optical device.
2. Description of the Related Art
Demand for higher bandwidth communications to end-users is growing. In this scenario, fiber optics is one of the most effective ways to deliver high bandwidth and fast speed, but fiber optics is rather expensive and difficult to be provided to end users. Typically, fiber optic cables are provided to end-users by burying the cables underground. However, installation of fiber optic cable underground is disruptive to neighborhoods and office areas because public streets and private properties are excavated to some degree. In addition, before excavating streets and properties, right-of-way contracts usually have to be negotiated, thereby wasting time and money. A further possible installation method consists in installing fiber optic cables in existing infrastructures, such as water pipes, sewage tubes and gas pipelines.
In particular situations, installing a fiber optical cable through existing gas pipelines can be very profitable under both the economical and practical point of view. It has been proved that the installation of optical cables through gas pipelines requires specific arrangements and changes in the existing gas pipelines, mainly consisting in the step of mounting input/output proper flanges for allowing the cable to be entered within, or extracted from, the pipeline.
An optical cable that is designed for being installed within a gas pipeline or the like should be provided with special characteristics and features. First, a similar cable should resist to radial pressure forces that are exerted by the fluid (gas) within the pipeline. As it is known, the typical tubes for transporting gas are pressurized up to about 70+100 bar, the pipes reaching the end user are pressurized at about 100+300 mbar and the intermediate distribution pipes are pressurized at 4+18 bar. The radial force resistance of an optical cable is generally obtained thanks to a proper sizing of the cable structure and thanks to a high filling of internal cavities that is obtained by applying jelly and other filling substances.
Second, a similar cable should resist to chemical attacks by the gas. In order to provide such a feature, it is known that a polyethylene-based protective jacket is suitable for the purpose.
Third, a similar cable should resist to gas longitudinal propagation in case of rupture of the external cable jacket. Also in this case, the resistance to gas longitudinal propagation is provided by jelly and other filling substances.
Fourth, a similar cable should resist to gas permeation both radially towards the inside of the cable and out of it. A rather high gas-proofing feature is obtained by a high-thickness high-density polythene outer jacket matched with a longitudinally applied metal tape having overlapping and welded/thermally-sealed edges, or alternatively with an extruded metal tube. The metal barrier complies with two main tasks. From one side, it does not allow that any hydrogen, possibly contained in the natural gas, comes into contact with the optical fibers resulting in a decreasing of the glass transmitting properties. From the other side, it does not allow that the natural gas, in case of entry within the cable, could permeate towards the external ambient where the cable is installed outside the gas pipeline.
Fifth, a similar cable should resist to abrasion and scoring against the walls of the pipelines. In this respect, a HDPE (High Density Polyethylene) sheath provides a low friction coefficient.
The Applicant has tackled the problem of realizing a telecommunication optical cable designed for being installed in gas pipelines or the like that, in addition to the above characteristics, is provided with a built-in gas leakage detecting device.
H.-D. Leppert et al., “Experience from the Field Installation of Optical Fiber Cables in Metro Gas Pipelines”, Proceedings of the 50th International Wire & Cable Symposium, pp. 747-752, describes in a detailed form a cable for installation through gas pipelines and specially developed I/O ports. It is also mentioned in the paper that the installation of an optical fiber cable inside the gas pipeline offers the additional possibility of implementing a leakage detection system. According to such a contribution, any fiber within the gas pipeline can be used for this application at no additional costs. The cable described in this document has the optical fibers enclosed in buffer tubes arranged around a central element. The buffer tubes are surrounded by a copolymer-coated aluminium tape and a high-density polyethylene sheath. The Applicant observes that, in this cable, the sensitivity of the optical fibers to leakages is poor, due to the various insulating layers provided around them.
EP-0 978 715 B1 discloses a device having at least one multimode fiber optic cable for laser light and a device to measure the propagation time and intensity of the backscattered light, to determine localized temperature anomalies along the cable. At least one single-mode optical fiber cable for message transfer forms a sheated composite arrangement with a compression and tensile resistant element. The cable is laid in the pipe with pressure-tight entry and exit guide connections. EP-0 978 715B1 also discloses a method for installing a fiber bundle cable arrangement for incorporation in the device and a unit for installing the fiber bundle cable arrangement.
DE-195 09 129 A1 concerns a method and device for checking and monitoring the state of tubes, containers, pipelines or the like for conveying liquid or gaseous fluid having temperatures which differ from that of the immediate environment. According to the DE-1 95 09 129 A1, the ambient temperature distribution is determined at least over sections along and/or about the periphery of the tubes, containers, pipelines or the like and/or in the area of the ground adjacent the tubes, containers, pipelines or the like, but outside the fluid-containing space which the latter surround. The ambient temperature distribution is determined by means of a distributed elongate temperature sensor, in particular a fiber-optic sensor cable for distributed temperature measurement. The detection of a local anomaly in the temperature distribution is indicative of a leak. The location, direction of diffusion and extent of the leak can be determined from the temperature distribution at each anomaly point or at each varying point of the anomaly. DE-195 09 129 A1 does not disclose a telecommunication optical cable with built-in leakage detecting device.
St. Groβwig et al., “Distributed fiber optical temperature sensing technique—a variable tool for monitoring tasks”, proceedings of the 8th International Symposium on temperature and thermal measurements in Industry and Science 19-21 Jun. 2001, pp 9-17, discloses fundamentals of the distributed fiber optical temperature sensing technique which is based on the RAMAN effect and the so-called Optical Time Domain Reflectometry (OTDR). The article describes applications of the distributed fiber optical temperature sensing method. As far as the leakage detection along high-pressure gas pipelines is concerned, it is mentioned that sensor fibers can also be integrated into telecommunication cables. However, the article does not disclose any cable structure that could be used for detecting leakages.